The propagation of acoustic waves in the earth is useful for land surveying operations. The waves are generated and the resultant refracted and reflected wave patterns are recorded with pickups at predetermined locations to study subsurface characteristics.
Prior art methods for mechanically generating acoustic waves in the earth teach that the generation of acoustic signals in the earth require the acceleration of the earth directly adjacent to the generating means. The methods all require a source of energy and, in general, rely on the force of gravity acting on a mass.
A prior art method uses a generating means consisting of a moving mass striking or impacting the earth. The simplest embodiment of this impacting method uses a manually manipulated mass, usually a hammer, to impact a plate placed on the surface of the earth. Although this manually actuated embodiment is widely used, the energy of the generated acoustic wave is severely limited by the strength of the individual manipulating the mass. Higher energy levels have been accomplished by utilizing a large impacting mass that is mechanically raised above the earth and released. The large mass is accelerated downward toward the earth by the force of gravity for impact. A simple embodiment of this mechanically assisted weight drop method is described by Neitzel in Geophysics, vol. XXIII, No. 1, p. 58, 1958. Another mechanically assisted weight drop method uses spring force in addition to the force of gravity to accelerate the mass toward the, earth as taught by Williams, U.S. Pat. No. 3,193,046. All of the mechanically assisted weight drop methods require a large, specialized impacting mass, as well as the associated support frame and lifting device. Additionally, a reaction mass is required if spring-assist mechanisms are used. Therefore, the embodiments of the impacting mass method either generates very low energy seismic signals or requires specialized, heavy apparatus to implement the method.
The effectiveness of the impacting mass technique can be improved by repeating the impact a number of times and summing the resulting data. In the simplest form of this summing technique the record obtained from a single impact is added to data obtained from previous impacts. This simplest summing method is very time consuming. An improved method for using a rapidly impacting device is taught by Barbier in U.S. Pat. No. 4,011,924, incorporated herein by reference. In this rapidly impacting method a pseudo-random sequence of impacts is generated by a mechanical device and the sequence is recorded and subsequently used to process the resulting seismic data using a modified correlation operation. The usefulness of the rapidly impacting apparatus described by Barbier is limited by the expense of a specialized impacting device and the potential for the sequence of impacts generated to be nonrandom and therefore resulting in data which is ringy (i.e., nonrandom resulting output oscillations) and of low resolution.
Another prior an method uses a mass to apply a continuous force to the earth. This force is mechanically varied with respect to time in a cyclical manner by acceleration of the mass. Typically the force applied to the earth is modulated in such a manner as to generate a swept-frequency sinusoidal seismic signal. Devices which use this method are generally referred to as vibrators and are used in conjunction with the Vibroseis method described by Geyer in the Journal of the Canadian Society of Exploration Geophysicists, vol. 6, pp. 39-58, incorporated herein by reference. An early embodiment of this method, which uses spinning eccentric weights to change the amplitude of the force applied to the earth, is taught by Doty et at., U.S. Pat. No. 2,668,128 and also described in Crawford, Doty and Lee; Geophysics vol. XXV, No. 1, p. 95, 1960. Subsequently, other embodiments of this method were developed using hydraulic-actuated vibrators to allow greater control of the generated signal. Yet another method of varying the force applied to the ground uses an electromagnetic technique to vary the force such as the apparatus taught by Cole in U.S. Pat. No. 3,313,370. All of the above-described vibrators require a dedicated actuating mechanism, large specialized reaction mass and typically specialized power supplies. Therefore, this modulated force method is expensive to implement and requires heavy, specialized equipment.
A number of seismic energy generating methods have been adapted for use with vehicles. They use the vehicle as a means of moving the energy generating means and often as a reaction mass against which the actuation mechanism such as springs or hydraulic actuators can act. Many weight drop type of sources have been mounted on vehicles. The vibrators described above are typically mounted on specialized vehicles. All of these adaptations require specialized modification of a standard vehicle or in some cases the construction of a special vehicle which adds considerable expense and can limit the utility of the vehicle for other applications.
Gibson in U.S. Pat. No. 4,301,888 teaches the use of the weight of a vehicle to supply a coupling force for a shear wave seismic energy generating apparatus. Although this approach eliminates the need for a specialized, dedicated reaction mass, the signal generating apparatus requires a specialized hydraulic cocking mechanism, is limited to shear wave generation, and requires a specialized power supply to restore the mechanism. Therefore, this method is expensive to implement and limited to shear wave seismic surveys.